1. Field of the Invention
The present invention relates to a device for driving a display panel that performs active driving of a luminescent element constituting a pixel by, for example, a TFT (Thin Film Transistor) and, more particularly, to a device for driving a display panel, which enable effectively applying a reverse bias voltage with respect to the luminescent element via a driving TFT.
2. Description of the Related Art
Development of a display that uses a display panel constructed of luminescent elements arranged in the form of a matrix has gone on being widely made. As a luminescent element that is used in such display panel, attention has been drawn toward an organic EL (electroluminescence) element wherein organic material is used in the luminescent layer. One of the reasons therefor is that, by using in a luminescent layer of the EL element an organic compound from which good luminescent property can be expected, the increase in the efficiency and that in the service life which can resist the practical use of the resulting EL element has made their progress.
As the display panel that uses such an organic EL element, two display panels have hitherto been proposed, one being a simple matrix type display panel wherein the EL elements are simply arranged in the form of a matrix and the other being an active matrix type display panel wherein to each of the EL elements arranged in the matrix form there has been added an active element consisting of a TFT. Compared with the former simple matrix type display element, the latter active matrix type display panel enables realizing low power consumption. In addition, it has the property of, for example, its being less in terms of the crosstalk between the pixels. It therefore is suitable especially for a display with a high degree of fineness that constitutes a large screen.
FIG. 1 illustrates an example of a circuit construction that corresponds to one pixel 10 in a conventional active matrix type display panel. Incidentally, the respective terminals, i.e. the source and the drain, of each of the TFTs that will be explained below, operationally, each function as the source or the drain depending on the voltage that is applied to the both terminals. Accordingly, in the following description, it is assumed that the expression “source” or “drain”, for convenience of the explanation, be handled as a name that is temporarily determined. Therefore, in the actual operational state in each of the circuit examples, there are also cases where that function is different (is reversed) from that corresponding to the name.
In FIG. 1, a gate G of a control TFT 11 is connected to a scanning line (the scanning line A1) and a source S is connected to a data line (the data line B1). Also, a drain D of the control TFT 11 is connected to a gate G of a drive TFT 12 and is also connected to one terminal of a capacitor 13 for holding electric charge. And a source S of the drive TFT 12 is connected to the other terminal of the capacitor 13 and is also connected to a common anode 16 formed within the panel. Also, a drain D of the drive TFT 12 is connected to an anode of an organic EL element 14 and a cathode of the organic EL element 14 is connected to a common cathode 17 that is formed within the panel.
FIG. 2 typically illustrates a state wherein the circuit construction that constitutes each pixel 10 illustrated in FIG. 1 is arrayed in a display panel 20. In each of the intersections of the respective control lines A1 to An and the respective data lines B1 to Bm, there is formed the pixel 10 having the circuit construction illustrated in FIG. 1. And, in the above-described construction, each source S of the drive TFTs 12 is respectively connected to the common anode 16 illustrated in FIG. 2 and the cathode of the respective EL elements 14 is connected to the common cathode 17 similarly illustrated in FIG. 2.
When, in this state, an “on” voltage is supplied to the gate G of the control TFT 11 of FIG. 1, the TFT 11 causes an electric current, corresponding to the voltage supplied from the data line to the source S, to flow from the source S to the drain D. Accordingly, during a time period in which the gate G of the TFT 11 has the voltage made “on”, the capacitor 13 is electrically charged, and the voltage is supplied to the gate G of the TFT 12. Thereby, the TFT 12 causes the electric current based on the gate voltage and the drain voltage to flow from the drain D into the common cathode 17 through the EL element 14 to thereby cause luminescence of the EL element 14.
Also, when the gate G of the TFT 11 has the voltage made “of f”, the TFT 11 becomes a so-called state of “cut-off”, with the result that the drain D of the TFT 11 becomes an open state. However, the drive TFT 12 has the voltage of its gate G held by the charge accumulated in the capacitor 13, thereby the drive current is maintained until the next scan is performed, thereby the luminescence of the EL element 14 is also maintained. Incidentally, since in the drive TFT 12 there exists the gate input capacitance, even if the capacitor 13 is not provided separately in particular, it is possible to cause the performance of the same operation as stated before.
In the conventional example illustrated in FIGS. 1 and 2, illustration is made of an example of display panel of a so-called “mono-chromatic luminescence” type, wherein, in every pixel, a serial circuit consisting of the drive TFT 12 and EL element 14 constituting a pixel is connected to between the common anode electrode 16 and the common cathode electrode 17. However, the device for driving a luminescent display panel that will be explained below can not only be of course adopted in a mono-chromatic luminescent display panel but can rather suitably be also adopted in, for example, a full-color type luminescent display panel that is equipped with respective luminescent pixels (sub-pixels) of R (red), G (green), and B (blue). Accordingly, in this case, without utilizing the common anode electrode 16 and the common cathode electrode 17 such as those described above, there is adopted a construction that is equipped with anode electrode lines or cathode electrode lines that are respectively separately provided correspondingly to the sub-pixels of R, G, and B.
Incidentally, it is known that the above-described organic EL element, saying from the electrical point of view, has a luminescent element having a diode characteristic and an electrostatic capacitance (parasitic capacitance) connected in parallel with respect thereto. Also, the organic EL element luminesces with a luminance that is almost proportionate to the magnitude of a forward-directional current having the diode characteristic. It is also empirically known that, in the above-described EL element, by sequentially applying a voltage of backward direction having no relevancy to the luminescence (backward bias voltage), the service life of the EL element can be extended.
In view thereof, in Patent document 1, there is disclosed a device for driving a luminescent display panel that is constructed in the way that, for example, within an addressing time period that designates the EL element that is to be lit up, so that a bias voltage of the polarity which is reverse to a forward-directional bias voltage is applied to the EL element. Also, in Patent document 2, there is also disclosed a device for driving a luminescent display panel in which, within a light-up time period of the EL element in the first sub-field (SF1) that starts from the terminating point in time of the addressing time period, there is set a time period (Tb) for simultaneously applying a reverse bias voltage to every EL element.
Patent document 1: Japanese Patent Application Laid-Open No. 2001-109432 (the paragraphs Nos. 0005 to 0007 described in FIGS. 5 and 6 and the like).
Patent document no. 2: Japanese Patent Application Laid-Open No. 2001-117534 (the paragraphs Nos. 0020 to 0023 described in FIGS. 8 and 10 and the like).
By the way, for performing active matrix driving of the above-described current drive-type luminescent element, it is said that a considerably high degree of electron mobility is necessary. For driving it, generally, there is used a polysilicon TFT. And, the relevant construction for performing that driving is generally as follows. Namely, in the drive TFT 12, by reason of the structure of the EL element 14, etc., there is used a P-channel type, and, in the control TFT 11, for ensuring a prescribed holding period by a small holding capacity, there is used an N-channel type that has a leak current that is small when turned off. In case where a thought is given of a construction wherein a combination of the above-described P-channel and N-channel TETs is adopted and, thereby, a reverse bias voltage can be applied to the EL element, the circuit constructions of the respective pixels such as those which are illustrated in, for example, FIGS. 3 to 7 can be taken up as examples. Incidentally, in FIGS. 3 to 7 that will be explained below, the elements that correspond to those illustrated in FIG. 1 are denoted by the same reference symbols.
First, the circuit construction of FIG. 3 is the one that is called a so-called “conductance control system” that is the same as the circuit construction explained in FIG. 1. And, by selecting the potential on the cathode side of the EL element 14 by a switch S1, a relevant construction is made so that a forward-directional voltage, or a reverse bias voltage, may be supplied to the EL element 14. In this case, in case where applying a forward-directional voltage to the EL element 14, the potential between the source of the drive TFT 12 and the cathode of the EL element 14 is set to be 15 V or so. Therefore, the potential of a VHanod illustrated in FIG. 3 is set to be 10 V while the potential of a VLcath is set to be −5 V or so. As a result of this, in a state where the switch S1 illustrated in FIG. 3 is in the illustrated state, it is possible to apply a forward-directional voltage to the EL element 14.
On the other hand, in case where supplying, in the circuit construction illustrated in FIG. 3, a reverse bias voltage to the EL element 14, the switch Si is changed over to a direction opposite to that illustrated, and, thereby, a VHbb is selected. In this case, the necessity arises of preparing, for the potential of the VHbb, a voltage source the potential of that is again higher than the potential of the VHanod, 10V. For instance, if attempting to apply a reverse bias voltage of 15 V to between the source of the drive TFT 12 and the cathode of the EL element 14, a voltage of 25 V becomes needed as the voltage level of VHbb.
Next, FIG. 4 illustrates an example of the 3-TFT type pixel construction for realizing the digital gradation. In the construction illustrated in FIG. 4, there is equipped an erasing TFT 21. By turning on that erasing TFT 21 during the light-in period of the EL element 14, it is possible to electrically discharge the electric charge of a capacitor 13. By this, it is possible to realize gradation driving for controlling the light-up period of the EL element 14. In this construction, as well, of FIG. 4, by selecting the potential on the cathode side of the EL element 14 by the switch S1, the construction is made so that a forward-directional voltage, or a reverse bias voltage, may be supplied to the EL element 14.
In the circuit construction, as well, illustrated in FIG. 4, if applying a reverse bias voltage of, for example, 15 V to between the source of the drive TFT 12 and the cathode of the EL element 14, it becomes necessary to use a power source for producing as the VHbb a voltage level of 25V.
Ensuring a power source voltage that has a level that is as relatively high as 25 V illustrated as the VHbb in the above-described way is not advisable when a consideration is given of loading the device into, for example, a portable equipment. Also, for light-up driving this type of active matrix panel, many power source voltages that include not only a signal for controlling the electric current that flows through the drive TFT but also a signal for controlling the control TFT become necessary. Especially, in case where considering loading into the portable equipment as described above, it is preferable, from the viewpoint of the actually mounting space and power consumption, that the number of the power source voltages be minimized and they be commonly used.
In view thereof, as illustrated in FIGS. 5 and 6, in addition to the changeover switch S1 (hereinafter referred to also as “the first switch”) there is further equipped a changeover switch S2 (hereinafter referred to also as “the second switch”). By doing so, in case where applying a forward-directional current to the EL element 14, the VHanod=10 V is applied via the second switch S2 to the source of the drive TFT 12 while to the cathode of the EL element 14 there is applied the VLcath=−5 V via the first switch S1. By doing so, a forward-directional voltage can be set to be 15V.
Also, in case where applying a reverse bias voltage to the EL element 14, by utilizing the both power sources, the VHanod=10 V and VLcath=−5V, the VLcath=−5 V can be applied to the source of the drive TFT 12 via the second switch S2. To the cathode electrode of the EL element 14 there can be applied a reverse bias voltage of 15V. By this, it is possible to omit the use of a power source the voltage level of that is fairly higher than that of other power sources, such as VHbb=25 V that was explained in FIGS. 3 and 4.
Furthermore, in case where ensuring a potential difference of 15 V as each of the forward-directional voltage and reverse bias voltage, this can be achieved by preparing the power sources of 10 V and 5 V in terms of the absolute value. Thereby, it becomes possible to drive the display panel with a power source circuit the voltage level of that is again lower.
By the way, in case where relevant control is performed by utilizing the switches S1 and S2 and, thereby, supplying each of the positive and negative power sources, by changing it over, when performing forward-directional driving and applying a reverse bias voltage, the following point in problem arises. As a result, there occurs the phenomenon that, especially at the time when applying a reverse bias voltage, it becomes difficult to effectively apply a reverse bias voltage with respect to the EL element 14.
The above-described points in problem will be explained by taking up the circuit construction illustrated in FIG. 5 as an example. Namely, in the circuit construction illustrated in FIG. 5, that the VHanod and VLcath are set in the way of the VHanod=10 V and VLcath=−5 V is as described before. In case where a consideration is given of a gate voltage of the TFT 12 that is necessary for performing on/off control of the drive TFT 12 when supplying a forward-directional current to the EL element 14, since the TFT 12 is a P-channel, a potential of 10 V at minimum becomes necessary for turning off the TFT 12. Also, for turning on the TFT 12, the earth potential (=0 V) that is a reference potential point can be utilized as is. Accordingly, as the data signal that is supplied to the source of the control TFT 11, the VHdata and VLdata can be set to be the VHdata=10 V and the VLdata=0 V.
Incidentally, in case where the earth potential that is the reference potential point can be utilized as the gate voltage for turning on the TFT 12 as described above, this technique is adopted, for example, when adjusting the luminous luminance of the EL element with the VHanod voltage and thereby performing digital gradation the gradation method of that is time gradation, etc. For instance, in case where adjusting the luminous luminance with a VLcont voltage and thereby performing digital gradation, or in case where performing analog gradation, an intermediate value between 0 V and 10 V is used as the gate voltage of the TFT 12. Accordingly, in the description that follows, an explanation will be given on the premise of a case where there is adopted the former construction of adjusting the luminance of the EL element with the VHanod and thereby performing digital gradation the gradation method of that is time gradation, etc.
Here, since the control TFT 11 is an N-channel as described before, in order to selectively supply the VHdata and VLdata signal to the gate of the drive TFT, it becomes necessary that a control voltage (VHcont) of 12 V prepared by adding a threshold voltage of at least 2 V to the VHdata=10 V be supplied to the gate of the control TFT 11. Also, during a non-scan period, the earth potential (=0 V) that is the reference potential point can be utilized as is with respect to the gate of the control TFT 11 to thereby enable turning off the control TFT 11. Accordingly, as the control line signal voltage that is supplied to the gate of the control TFT 11, preferably, it is set to be the VHcont=12 V and VLcont=0 V.
Here, at the time when changing over the applied state of the EL element 14 from a state where a forward-directional voltage is being applied to the EL element 14 to a state where a reverse bias voltage is applied thereto, a resetting operation of electrically discharging the electric charge of the capacitor 13 is executed. Namely, in a state where a forward-directional voltage is applied, a voltage of VHanod=10 V is being applied to one terminal (a) of the capacitor 13. Therefore, when supplying a voltage of VHcont=12 V to the control line and, at this time, supplying a voltage of VHdata=10 V to the data line, a voltage of 10 V (VHdata) is applied to the other terminal (b) of the capacitor 13. Accordingly, at that moment, the voltages at the both terminals of the capacitor 13 become equal in potential, whereby the electric charge is discharged (reset). Thereafter, a voltage of the VLcont=0 V is supplied, thereby the control TFT 11 is turned off.
Subsequently, the changeover switches S1 and S2 illustrated in FIG. 5 are each changed over to a direction opposite to that illustrated therein. And, a voltage of VLcath=−5 V is supplied to the source of the drive TFT 12 while a voltage of VHanod=10 V is supplied to the cathode of the EL element 14. At this moment, −5 V is led into the terminal (b) via the capacitor 13 the charge of that is in a state of being electrically discharged. At this moment, −5 V is also led into the drain, as well, of the control TFT 11, whereby the drain of the control TFT 11 the voltage of that has been sufficiently made low as compared with the gate voltage thereof substantially functions as the source. Therefore, since the control TFT 11 is an N-channel, it becomes instantaneously turned on because of the relationship biased as described before. Therefore, via the control TFT 11, the gate potential of the drive TFT 12 is raised from −5 V and, in extreme cases, sometimes, is raised up to a level of around +10V.
Also, in the drive TFT 12, because of the above-described changeover of the changeover switches S1 and S2, the source and the drain have their functions inverted. Thereby, a gate voltage that is approximate to the source potential (VHanod=10V) attained by the function being inverted is applied to the gate of the drive TFT 12. As a result of this, the drive TFT 12 is brought to a state of its being turned off. As a result of this, it becomes impossible to effectively apply a reverse bias voltage to the EL element 14. Therefore, the problem remains that the effect of extending the service life of the EL element becomes halved.
On the other hand, the applicant of this application applied, as Japanese Patent Application No. 2002-230072, for a patent on a circuit construction wherein a diode is connected in parallel to the drive TFT; and, by utilizing the action of the diode that becomes electrically conductive when applying a reverse bias voltage, a reverse bias voltage is effectively applied to the EL element 14. FIG. 7 illustrates a circuit construction wherein the diode 18 is added to the circuit construction illustrated in FIG. 6. According to the construction illustrated in FIG. 7, in case where the switches S1 and S2 have each been changed over to a state opposite to that which is illustrated and a reverse bias voltage has been applied to the EL element 14, the diode 18 becomes electrically conductive. By this, it is possible to effectively apply a reverse bias voltage to the EL element 14.
However, according to the circuit construction illustrated in FIG. 7, in a state where a reverse bias voltage is being applied to the EL element 14, because the TFT 21 and TFT 11 are each an N channel, each of them is turned on. Resultantly, there arises the inconvenience that short-circuiting between the VLcath and the VHdata or VLdata occurs.